Resin transfer molding is a closed molding operation for forming reinforced composite parts, such as chairs, automobile parts, aircraft components and surfboard fins. It involves placing a reinforcement structure within a mold cavity, wherein the mold cavity has a shape of the desired composite part. The mold is then closed and a matrix resin is injected into the mold under pressure such that the resin material substantially completely impregnates the reinforcement structure. After resin impregnation has been effected, the resin is cured, such as by heating the mold. Thereafter, the resultant composite part is removed from the mold.
One known reinforcement structure for use in forming reinforced composite parts comprises first and second outer layers of reinforcement fibers, such as discrete glass fibers. Located between the outer layers is a flexible core layer comprising elongated fibers, such as synthetic fibers. During a resin transfer molding operation, the core layer is substantially impregnated with resin material. Once cured, the resin material is typically dense, has poor to average shear strength during bending or flexure and has a poor to average elastic modulus. Consequently, due to the high content of matrix resin material in the core layer, composite parts formed with this reinforcement structure typically have a center or core section which is substantially high in weight, a shear strength during bending or flexure similar to that of the matrix resin, and an elastic modulus similar to that of the matrix resin. It is also noted that resin shrinkage increases as the matrix resin content in a composite part increases.
It would be desirable to have a reinforcement structure comprising first and second outer layers of reinforcement fibers and a flexible core layer, and wherein, after matrix resin impregnation of the reinforcement structure, a composite part results having a core section which is light weight, and has improved mechanical properties over those of a core section of a composite part having incorporated therein the prior art reinforcement structure noted above.